Surface-relief transmission diffraction grating elements.
U.S. Pat. No. 5,917,625 discloses, in FIG. 26, a multiplexing/demultiplexing device which utilizes a transmission type diffraction grating 140 disposed in front of a reflection mirror 115. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
A device similar to that disclosed in U.S. Pat. No. 5,917,625 is claimed in U.S. patent application Ser. No. 09/193,289. This United States patent application is discussed in Columns 1 and 2 of U.S. Pat. No. 6,108,471, the disclosure of which is also hereby incorporated by reference into this specification.
The U.S. Ser. No. 09/193,289 patent application claimed an optical multiplexing and demultiplexing device comprising a fiber mounting assembly for securing a plurality of optical fibers, collimating and focusing lens, a transmissive grating including a diffractive element formed from a photosensitive medium, and a mirror for receiving at least one beam coming from at least one of the plurality of optical fibers via the lens and the grating and for reflecting one or more of the beams back through the grating and the lens to at least one of the optical fibers. The photosensitive medium disclosed in U.S. Ser. No. 09/193,289 was dichromate gelatin (DCG).
DCG transmissive gratings are transmission volume phase gratings and, thus, the diffracting grating layer in these grating elements tend to have a thickness which is significantly larger than the corresponding layer in surface-relief grating elements. Thus, such gratings tend to change their optical properties with changes in temperature more than that achieved with surface-relief gratings fabricated on low thermal expansion substrate materials. Furthermore, because of their thickness, DCG gratings tend to be more sensitive to angular alignment issues than are surface-relief gratings; and, because of such alignment issues, the DCG gratings are not readily usable for broad spectrum wavelength beam applications that require equal diffraction efficiency for all of the spectral components of the beam. For fiber-optic telecommunication systems having wavelength channel signals over the spectrum range of from 1280 to 1620 nanamometers, the DCG grating-based devices tend not to be as useful for this system application as surface-relief grating-based devices.
Surface-relief reflection grating elements are well known to those skilled in the art, and their diffracting grating layer is substantially thinner than that incorporated in DCG grating elements; consequently, they do not suffer from many of the disadvantages of DCG gratings. The properties of surface-relief reflection grating elements are disclosed in Christopher Palmer""s xe2x80x9cDiffraction Grating Handbook,xe2x80x9d Fourth Edition (Richardson Grating Laboratory, Rochester, N.Y. 14605). Reference also may be had to a paper by E. G. Loewen et al. entitled xe2x80x9cGrating efficiency theory as it applies to blazed and holographic gratings,xe2x80x9d (Applied Optics, Volume 16, page 2711, October, 1977)
While surface-relief transmission grating elements are not as well known or used as surface-relief reflection grating elements, they are commercially available from Holotek LLC of Henrietta, N.Y. Surface-relief transmission grating elements have the same advantages relative to DCG gratings that surface-relief reflection grating elements have; and they provide even more advantages than surface-relief reflection grating elements when used in fiber-optic communication devices. In particular, they can provide higher wavelength dispersion power while still achieving essentially equal diffraction efficiency values for S and P polarized optical components. However, when the grating surfaces of prior art surface-relief transmission grating elements are subjected to a temperature of 85 degrees centigrade at a relative humidity of 85 percent for two hours or less, the grating surfaces are degraded until the grating structure disappears. This test is often referred to as the xe2x80x9cBellcore High Temperature High Humidity Storage Test for Fiber Optic Devices.xe2x80x9d
It is an object of this invention to provide a surface-relief transmission grating with improved durability when subjected to the Bellcore High Temperature High Humidity Storage test conditions.
It is another object of this invention to have such improved surface-relief transmission gratings have greater than 70 percent diffraction efficiency values for S and P polarized optical components while achieving essentially equal diffraction efficiency values for these polarization components, that is, the S and P polarizations have diffraction efficiency values within about 5 percent of each other.
It is yet anther object of this invention to have such improved surface-relief transmission grating use a low thermal expansion substrate material and, thereby achieve a change in grating line spacing that is in an accept range when the grating is used over the 70 degree centigrade temperature range specified for fiber-optic communication devices.
It is yet anther object of this invention to have such improved surface-relief transmission grating surface be encapsulated and, thereby protect the grating surface from being damaged due to handling and cleaning of the grating element, as well as, from contaminants, liquids or solvent vapors that could damage the grating surface.
It is yet another object of this invention to provide devices incorporating such improved surface-relief transmission grating.
It is yet another object of this invention to provide grating-based devices having higher wavelength dispersion power while providing essentially equal radiometric throughput efficiency values for S and P polarized optical components, that is, the S and P polarizations have device radiometric throughput efficiency values equal to within about 5 percent of each other.
In accordance with this invention, there is provided a surface-relief transmission diffraction grating which, after having been subjected to a test condition of 85 degrees centigrade and a relative humidity of 85 percent for at least 500 hours, has diffraction efficiency performance within 6 percent of that achieved before being subjected to these test conditions.
Also, in accordance with this invention this surface-relief transmission grating is incorporated in wavelength selection devices, such as used in fiber-optic communication systems.